The invention relates to an automatic belt tensioner with a torsional spring that biases the position of a pivot-arm to which a belt engaging pulley is attached, but more particularly, the invention relates to a tensioner of the "ZED" type where a radial center plane for the pulley is laterally offset from an axis for a pivot about which the pivot-arm oscillates.
Automatic belt tensioners have a pivot-arm that pivots relative to a base. A pulley is rotatably mounted to the pivot-arm and a spring between the pivot-arm and base biases the position of the pivot-arm to place the pulley against an endless belt of a belt drive system to tension the belt while receiving a belt induced load. In some applications, it is preferred to have a bushing for the pivot to be in a common radial plane with the pulley to simplify or reduce loads that must be carried by the bushing. Within this category of tensioners, there is a type that has a bushing that provides both bearing support and damping to pivoted oscillatory movements of a pivot-arm. Such a tensioner is disclosed in U.S. Pat. No. 4,723,934 (B 1 4,723,934). Another example of a tensioner that has a bushing that provides both bearing support and damping to pivotal movements of a pivot-arm is disclosed in U.S. Pat. No. 5,478,285.
An example of a "ZED" type tensioner is disclosed in U.S. Pat. No. 4,473,362 where the tensioner has a pivot-arm attached to an off-set annular pivot member that supports the pivot-arm and oscillates on a post secured to a base. A torsion spring is used where one end of the spring is constrained at two points by a connection to the pivot-arm, and the other end of the spring is constrained by one connection to the base and one connection by way of a damping mechanism that includes a coil of the spring that presses against a sleeve which in turn presses against the annular pivot member. A single sleeve-type bushing located radially inward of the spring and positioned between the post and annular member, has a bearing surface that supports the annular member with its interconnected pivot-arm. The pivot bushing in connection with the damping mechanism, damps oscillatory movements of the pivot-arm. A problem with this type of tensioner is that the amount of available damping is somewhat limited because the pivot bushing and damping mechanism are located radially inward of the torsion spring and operate at a radius that is substantially less than the length of the pivot-arm.
Another example of a "ZED" type tensioner is disclosed in U.S. patent application Ser. No. 08/828,216 and has: a pivot-arm, a pivot-pin connected on one side to the pivot-arm, a base member of generally "tube-pan" shape holding at least one low friction bushing that supports the pivot-pin, a torsional spring with one end constrained and attached to the pivot-arm and another end constrained and attached to the base. The tensioner has a damping mechanism where an end of the spring is operatively connected to a damping mechanism that includes a shoe with an outside friction surface that rubs against an inside wall of the base member to effect damping. A characteristic associated with tensioners of this type is that substantially all of the damping is by way of the damping mechanism; very little damping is generated at the pivot bushing in response to a belt load received at a pulley.